1. Field of the Invention
The present invention relates in general to a fluorescence image display apparatus, and in particular to a fluorescence image display apparatus for measuring the fluorescent light emitted from a target subject upon the irradiation thereof by an excitation light and displaying as an image the data relating to said target subject.
2. Description of the Related Art
Fluorescent light detection apparatuses have been proposed that make use of the fact that the intensity of the fluorescent light emitted from a normal tissue differs from the intensity of the fluorescent light emitted from a diseased tissue when a target subject (i.e., a living tissue) is irradiated by an excitation light within an excitation wavelength range of the intrinsic fluorophores of the target subject, wherein, by detecting the fluorescent light emitted from a target subject upon irradiation thereof by an excitation light within a predetermined wavelength range, the location and range of penetration of a diseased tissue is discerned.
Normally, when a target subject is irradiated by an excitation light, because a high-intensity fluorescent light is emitted from a normal tissue, as shown by the solid line in FIG. 26, and a weak-intensity fluorescent light is emitted from a diseased tissue, as shown by the broken line in FIG. 26, by measuring the intensity of the fluorescent light emitted from the target subject, it can be determined whether the target subject is in a normal or a diseased state. These types of fluorescence image display apparatuses are in many cases provided built in to the configuration of an endoscope for insertion into a body cavity of a patient, a colposcope, or a surgical microscope.
However, for cases in which the intensity of the fluorescent light emitted from a target subject upon the irradiation thereof by the excitation light is to be displayed as an image, because there is unevenness on the surface of a target subject, the intensity of the excitation light irradiating the target subject is not of a uniform intensity. Further, although the intensity of the fluorescent-light emitted from the target subject is substantially proportional to the intensity of the excitation light, the intensity of the aforementioned excitation light becomes weaker in inverse proportion to the square of the distance between the excitation light and the target subject. Therefore, there are cases in which the fluorescent-light received from a diseased tissue located at a position closer to the excitation light source than a normal tissue is of a higher intensity than the fluorescent-light received from aforementioned normal tissue, and the state of the tissue of the target subject cannot be accurately discerned based solely on the data relating to the intensity of the fluorescent-light received from the target subject upon the irradiation thereof with an excitation light.
In order to remedy the problems described above, methods such as that described in the specification of U.S. Pat. No. 5,647,368, wherein by colorizing and synthesizing a fluorescence image obtained by irradiating a target subject with an excitation light having a wavelength in the wavelength range near 500 nm, upon which the intensity of the fluorescent light emitted from the target subject changes by a large degree depending on the tissue state of the target subject, with a fluorescence image obtained by irradiating the target subject with an excitation light having a frequency in the wavelength range near 630 nm, upon which the intensity of the fluorescent light emitted from the target subject exhibits no change depending on the tissue state of the target subject, to obtain a colorized synthesized image; when said colorized synthesized image is displayed, the tissue state of the target subject can be accurately discerned based on the visually recognizable color thereof, have been proposed.
Further, there has been proposed, as described in Japanese Unexamined Patent Publication No. 2001-157658 (not patented), a method of colorizing and displaying two types of fluorescence images based on the irradiation of a target subject with two different wavelengths of excitation light (a narrow band excitation light having a wavelength near 480 nm, and a wide band excitation light having a wavelength within the wide band of 430-730 nm). According to this method, because the band of the fluorescent light is wider in comparison to the method proposed in the aforementioned U.S. Pat. No. 5,647,368, the S/N ratio of the images obtained by additive color mixture can be improved, and because the intensity of the fluorescence images changes by a large amount in accordance with the tissue state of the target subject for both bands, the color change corresponding to the change in the tissue state can be made more distinct, whereby the distinguishability of the tissue state is improved a level.
However, according to the above described methods of displaying colorized synthesized images, the color of the image to be displayed is regulated by a chromaticity, which is determined in correspondence to the of the ratio of the two types of fluorescence images that are to be color added and mixed, and a luminosity, which is determined in correspondence to the intensity of the fluorescence images. Because the chromaticity of the image to be displayed is determined in correspondence to the ratio of the two types of fluorescence images that are to be color added and mixed, the chromaticity of the image that is to be displayed, which corresponds to the tissue state of the target subject, can be determined at once. However, because the fluorescent light intensity differs according to the distance between the light source and the target subject, the luminosity of the image to be colorized differs according to the aforementioned distance. Here, these types of fluorescence image display apparatuses are in many cases provided in the configuration of an endoscope for insertion into a body cavity of a patient, a colposcope, or a surgical microscope; because the objective of the use thereof is the measurement of an internal portion of a body cavity, the distance between the target subject and any of said apparatuses is between several to 50 mm. Therefore, if the distance between the distal end of the insertion portion that is inserted into the body cavity of a patient and the target subject changes, the intensity of the fluorescent light changes; as a result, the luminosity of the synthesized image changes. If the luminosity of the colorized synthesized image changes in this manner, because the color appearing in the displayed color added and mixed image will be recognized as a different color even if the chromaticity thereof is the same, there is a fear that even though tissues have the same tissue state, said tissues will be discerned to have different tissue states. On the other hand, for cases in which the luminosity of the image to be colorized is low, there are cases in which even though a chromaticity is different, the color in the displayed colorized synthesized image cannot be recognized to be different, giving rise to a fear that a diseased portion might be overlooked.